subscriber: Add SubscriberBuilder/Layer accessors (tokio-rs#1871)

## Motivation

`SubscriberBuilder`s and `Layer`s configured with custom event/field
formatters do not provide any means of accessing or mutating those
formatters. Any configuration that needs to be done must be done before
setting them on the builder/layer. This is frustrating as it makes it
difficult to provide a pre-configured API akin to
`tracing_subscriber::fmt()` along with accessors like `.compact()` that
modify the formatter.

## Solution

Add accessors `.map_event_format()` and `.map_fmt_fields()` to
`SubscriberBuilder` and `Layer` that map the existing formatter through
a closure. This allows the closure to modify it or to derive a new
formatter from it with a different type.

Also add a `.map_writer()` method that does the same thing for the
`MakeWriter`, to round out the accessors for the various type
parameters.

The filter type is currently restricted to just `LevelFilter` or
`EnvFilter` and so this does not add a corresponding `.map_filter()`.
That can be added later if we add the ability to attach arbitrary
filters.

Also fix some minor docs issues that were spotted as part of
implementing this.

Fixes tokio-rs#1756

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -87,8 +87,8 @@ where
     N: for<'writer> FormatFields<'writer> + 'static,
     W: for<'writer> MakeWriter<'writer> + 'static,
 {
-    /// Sets the [event formatter][`FormatEvent`] that the layer will use to
-    /// format events.
+    /// Sets the [event formatter][`FormatEvent`] that the layer being built will
+    /// use to format events.
     ///
     /// The event formatter may be any type implementing the [`FormatEvent`]
     /// trait, which is implemented for all functions taking a [`FmtContext`], a
@@ -122,11 +122,40 @@ where
             _inner: self._inner,
         }
     }
+
+    /// Updates the event formatter by applying a function to the existing event formatter.
+    ///
+    /// This sets the event formatter that the layer being built will use to record fields.
+    ///
+    /// # Examples
+    ///
+    /// Updating an event formatter:
+    ///
+    /// ```rust
+    /// let layer = tracing_subscriber::fmt::layer()
+    ///     .map_event_format(|e| e.compact());
+    /// # // this is necessary for type inference.
+    /// # use tracing_subscriber::Layer as _;
+    /// # let _ = layer.with_subscriber(tracing_subscriber::registry::Registry::default());
+    /// ```
+    pub fn map_event_format<E2>(self, f: impl FnOnce(E) -> E2) -> Layer<S, N, E2, W>
+    where
+        E2: FormatEvent<S, N> + 'static,
+    {
+        Layer {
+            fmt_fields: self.fmt_fields,
+            fmt_event: f(self.fmt_event),
+            fmt_span: self.fmt_span,
+            make_writer: self.make_writer,
+            is_ansi: self.is_ansi,
+            _inner: self._inner,
+        }
+    }
 }
 
 // This needs to be a seperate impl block because they place different bounds on the type parameters.
 impl<S, N, E, W> Layer<S, N, E, W> {
-    /// Sets the [`MakeWriter`] that the [`Layer`] being built will use to write events.
+    /// Sets the [`MakeWriter`] that the layer being built will use to write events.
     ///
     /// # Examples
     ///
@@ -142,9 +171,6 @@ impl<S, N, E, W> Layer<S, N, E, W> {
     /// # use tracing_subscriber::Layer as _;
     /// # let _ = layer.with_subscriber(tracing_subscriber::registry::Registry::default());
     /// ```
-    ///
-    /// [`MakeWriter`]: ../fmt/trait.MakeWriter.html
-    /// [`Layer`]: ../layer/trait.Layer.html
     pub fn with_writer<W2>(self, make_writer: W2) -> Layer<S, N, E, W2>
     where
         W2: for<'writer> MakeWriter<'writer> + 'static,
@@ -201,6 +227,39 @@ impl<S, N, E, W> Layer<S, N, E, W> {
             ..self
         }
     }
+
+    /// Updates the [`MakeWriter`] by applying a function to the existing [`MakeWriter`].
+    ///
+    /// This sets the [`MakeWriter`] that the layer being built will use to write events.
+    ///
+    /// # Examples
+    ///
+    /// Redirect output to stderr if level is <= WARN:
+    ///
+    /// ```rust
+    /// use tracing::Level;
+    /// use tracing_subscriber::fmt::{self, writer::MakeWriterExt};
+    ///
+    /// let stderr = std::io::stderr.with_max_level(Level::WARN);
+    /// let layer = fmt::layer()
+    ///     .map_writer(move |w| stderr.or_else(w));
+    /// # // this is necessary for type inference.
+    /// # use tracing_subscriber::Layer as _;
+    /// # let _ = layer.with_subscriber(tracing_subscriber::registry::Registry::default());
+    /// ```
+    pub fn map_writer<W2>(self, f: impl FnOnce(W) -> W2) -> Layer<S, N, E, W2>
+    where
+        W2: for<'writer> MakeWriter<'writer> + 'static,
+    {
+        Layer {
+            fmt_fields: self.fmt_fields,
+            fmt_event: self.fmt_event,
+            fmt_span: self.fmt_span,
+            is_ansi: self.is_ansi,
+            make_writer: f(self.make_writer),
+            _inner: self._inner,
+        }
+    }
 }
 
 impl<S, N, L, T, W> Layer<S, N, format::Format<L, T>, W>
@@ -481,6 +540,36 @@ impl<S, N, E, W> Layer<S, N, E, W> {
             _inner: self._inner,
         }
     }
+
+    /// Updates the field formatter by applying a function to the existing field formatter.
+    ///
+    /// This sets the field formatter that the layer being built will use to record fields.
+    ///
+    /// # Examples
+    ///
+    /// Updating a field formatter:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::field::MakeExt;
+    /// let layer = tracing_subscriber::fmt::layer()
+    ///     .map_fmt_fields(|f| f.debug_alt());
+    /// # // this is necessary for type inference.
+    /// # use tracing_subscriber::Layer as _;
+    /// # let _ = layer.with_subscriber(tracing_subscriber::registry::Registry::default());
+    /// ```
+    pub fn map_fmt_fields<N2>(self, f: impl FnOnce(N) -> N2) -> Layer<S, N2, E, W>
+    where
+        N2: for<'writer> FormatFields<'writer> + 'static,
+    {
+        Layer {
+            fmt_event: self.fmt_event,
+            fmt_fields: f(self.fmt_fields),
+            fmt_span: self.fmt_span,
+            make_writer: self.make_writer,
+            is_ansi: self.is_ansi,
+            _inner: self._inner,
+        }
+    }
 }
 
 impl<S> Default for Layer<S> {

tracing-subscriber/src/fmt/mod.rs

@@ -923,7 +923,7 @@ where
 }
 
 impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
-    /// Sets the Visitor that the subscriber being built will use to record
+    /// Sets the field formatter that the subscriber being built will use to record
     /// fields.
     ///
     /// For example:
@@ -1021,7 +1021,7 @@ impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
     /// subscriber.
     ///
     /// If the max level has already been set, or a [`EnvFilter`] was added by
-    /// [`with_filter`], this replaces that configuration with the new
+    /// [`with_env_filter`], this replaces that configuration with the new
     /// maximum level.
     ///
     /// # Examples
@@ -1044,8 +1044,8 @@ impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
     ///     .finish();
     /// ```
     /// [verbosity level]: https://docs.rs/tracing-core/0.1.5/tracing_core/struct.Level.html
-    /// [`EnvFilter`]: ../filter/struct.EnvFilter.html
-    /// [`with_filter`]: #method.with_filter
+    /// [`EnvFilter`]: struct@crate::filter::EnvFilter
+    /// [`with_env_filter`]: fn@Self::with_env_filter
     pub fn with_max_level(
         self,
         filter: impl Into<LevelFilter>,
@@ -1057,8 +1057,26 @@ impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
         }
     }
 
-    /// Sets the function that the subscriber being built should use to format
-    /// events that occur.
+    /// Sets the [event formatter][`FormatEvent`] that the subscriber being built
+    /// will use to format events that occur.
+    ///
+    /// The event formatter may be any type implementing the [`FormatEvent`]
+    /// trait, which is implemented for all functions taking a [`FmtContext`], a
+    /// [`Writer`], and an [`Event`].
+    ///
+    /// # Examples
+    ///
+    /// Setting a type implementing [`FormatEvent`] as the formatter:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::fmt::format;
+    ///
+    /// let subscriber = tracing_subscriber::fmt()
+    ///     .event_format(format().compact())
+    ///     .finish();
+    /// ```
+    ///
+    /// [`Writer`]: struct@self::format::Writer
     pub fn event_format<E2>(self, fmt_event: E2) -> SubscriberBuilder<N, E2, F, W>
     where
         E2: FormatEvent<Registry, N> + 'static,
@@ -1085,8 +1103,6 @@ impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
     ///     .with_writer(io::stderr)
     ///     .init();
     /// ```
-    ///
-    /// [`MakeWriter`]: trait.MakeWriter.html
     pub fn with_writer<W2>(self, make_writer: W2) -> SubscriberBuilder<N, E, F, W2>
     where
         W2: for<'writer> MakeWriter<'writer> + 'static,
@@ -1127,6 +1143,82 @@ impl<N, E, F, W> SubscriberBuilder<N, E, F, W> {
             inner: self.inner.with_writer(TestWriter::default()),
         }
     }
+
+    /// Updates the event formatter by applying a function to the existing event formatter.
+    ///
+    /// This sets the event formatter that the subscriber being built will use to record fields.
+    ///
+    /// # Examples
+    ///
+    /// Updating an event formatter:
+    ///
+    /// ```rust
+    /// let subscriber = tracing_subscriber::fmt()
+    ///     .map_event_format(|e| e.compact())
+    ///     .finish();
+    /// ```
+    pub fn map_event_format<E2>(self, f: impl FnOnce(E) -> E2) -> SubscriberBuilder<N, E2, F, W>
+    where
+        E2: FormatEvent<Registry, N> + 'static,
+        N: for<'writer> FormatFields<'writer> + 'static,
+        W: for<'writer> MakeWriter<'writer> + 'static,
+    {
+        SubscriberBuilder {
+            filter: self.filter,
+            inner: self.inner.map_event_format(f),
+        }
+    }
+
+    /// Updates the field formatter by applying a function to the existing field formatter.
+    ///
+    /// This sets the field formatter that the subscriber being built will use to record fields.
+    ///
+    /// # Examples
+    ///
+    /// Updating a field formatter:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::field::MakeExt;
+    /// let subscriber = tracing_subscriber::fmt()
+    ///     .map_fmt_fields(|f| f.debug_alt())
+    ///     .finish();
+    /// ```
+    pub fn map_fmt_fields<N2>(self, f: impl FnOnce(N) -> N2) -> SubscriberBuilder<N2, E, F, W>
+    where
+        N2: for<'writer> FormatFields<'writer> + 'static,
+    {
+        SubscriberBuilder {
+            filter: self.filter,
+            inner: self.inner.map_fmt_fields(f),
+        }
+    }
+
+    /// Updates the [`MakeWriter`] by applying a function to the existing [`MakeWriter`].
+    ///
+    /// This sets the [`MakeWriter`] that the subscriber being built will use to write events.
+    ///
+    /// # Examples
+    ///
+    /// Redirect output to stderr if level is <= WARN:
+    ///
+    /// ```rust
+    /// use tracing::Level;
+    /// use tracing_subscriber::fmt::{self, writer::MakeWriterExt};
+    ///
+    /// let stderr = std::io::stderr.with_max_level(Level::WARN);
+    /// let layer = tracing_subscriber::fmt()
+    ///     .map_writer(move |w| stderr.or_else(w))
+    ///     .finish();
+    /// ```
+    pub fn map_writer<W2>(self, f: impl FnOnce(W) -> W2) -> SubscriberBuilder<N, E, F, W2>
+    where
+        W2: for<'writer> MakeWriter<'writer> + 'static,
+    {
+        SubscriberBuilder {
+            filter: self.filter,
+            inner: self.inner.map_writer(f),
+        }
+    }
 }
 
 /// Install a global tracing subscriber that listens for events and

tracing-subscriber/src/layer/mod.rs

@@ -857,7 +857,7 @@ where
     /// per-layer filtering.
     ///
     /// [`Filtered`]: crate::filter::Filtered
-    /// [plf]: #per-layer-filtering
+    /// [plf]: crate::layer#per-layer-filtering
     #[cfg(all(feature = "registry", feature = "std"))]
     #[cfg_attr(docsrs, doc(cfg(all(feature = "registry", feature = "std"))))]
     fn with_filter<F>(self, filter: F) -> filter::Filtered<Self, F, S>